Another possible fix is to SSH into root/private/ect/apt/sources.list.d and remove all entries except saurik.list.
This will delete all your manually added repos. (Thanks to neyzcorp for this fix)

Cydia Won't Add a Repo:

There are very few possible explanations, here are a few:
1) The repo is down, check the forums for more information.
2) You typed it wrong, many people forget to put the / at the end of a repo url.
3) You aren't connect to a network ... pretty self explanatory.

Source Error:

A common error message would be Bzip2 error or the fact that a source is not functioning correctly.

IF A REPO ONLY ACCEPTS ONE DEVICE PER ACCOUNT YOU WILL GET THIS MESSAGE IF YOU ADD MORE THAN ONE DEVICE (our repo is like this)

If you cannot remove the source from Cydia or the problem persists, try this:

1) SSH into your device.
2) Navigate to the /etc/apt/ folder.
3) Backup the cydia.list OR sources.list.d file.
4) Edit the cydia.list OR sources.list.d file with a text editor and remove the entry related to the problematic source.
5) Reboot.

Crash After "Reloading Data":

If Cydia is crashing after the Reloading Data message then try this:
(If you don't have terminal installed, use terminal through SSH)

1) Open Mobile Terminal/Terminal via SSH.
2) Type:

su
alpine
dpkg --configure -a

3) Reboot.

I am unsure that this is a valid fix but if you try it let me know the outcome please.

Package Header Error:

If on startup, Cydia says there's an error with package headers do the following to fix it.

1) Open terminal and type:

su
alpine
dpkg --a configure

2) Respring just to make sure it reloads the data.

Icon is not Visible:

If your Cydia icon disappeared after you installed a new package and restarting doesn't get it back, follow these steps:

1) SSH into /applications/cydia.app
2) Copy the icon.png to your computer.
3) Delete icon.png from the device
4) Respring and a basic, white icon will appear for Cydia.
5) Copy the icon.png back to /applications/cydia.app
6) Respring and the normal icon should be there.

Cyder:

Cyder is a progam for your computer that downloads and transfers packages/sources to your device.
It also has other useful feature which may fix problems such as clearing the cache.
To get it go here:

A New Cache Fix:

If you are having problems with the cache and want to clear it try this new tool. There are 2 things you must install first though.

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